The present invention relates to highly-filled homogeneous high-viscosity pastes, and to a device for, and method of, de-aerating and injecting such highly-filled pastes, particularly for the fabrication of explosive devices such as hollow charge munitions, linear charges, and magnetic generators.
Shaped charges are traditionally manufactured from pressed plastic bonded explosives (PBX). The pressing technology has profound advantages, mainly, high solid content, which is associated with the high energy of the shaped charge, and homogeneity, which is associated with the precision of the shaped charge. Nevertheless, pressed charges have also some profound drawbacks: poor mechanical properties, relatively high vulnerability, and high cost due to the requisite machining.
The performance of highly-filled explosive devices is strongly dependent on the geometry and homogeneity of the device. Another important factor in determining the performance and sensitivity of an explosive device is the relative density measured by the percent of the theoretical maximum density (TMD) of the explosive composition. The TMD of a blended composition is determined by the specific gravities and by the relative proportions in which the ingredients are present.
The actual measured value of the composition density, on average, is always lower than, but may closely approach, the TMD. With modern mixing techniques and vacuum equipment, the actual density of an explosive composition can approach 98-99% or more of the composition's theoretical density, provided that the mixture is a low-viscosity mixture (i.e., below 50, and typically, below 20 KPa·seconds).
By sharp contrast, in highly-filled pastes, the high-viscosity (above 100 KPa·seconds, and especially above 500-1000 KPa·seconds) of the paste makes the residual air (entrapped during mixing and transport operations), extremely difficult to remove. For such pastes, there is a significant difference between the actual density and the TMD. The presence of entrapped air must be minimized before the material is injected in order to avoid undesired air pockets in the injected products.
It must be emphasized that increasing the relative density (i.e., the percent of the TMD) of an explosive composition is highly desirable because even small increases in the relative density significantly increase the explosive “punch”—specifically, higher velocity of detonation (VOD) and increased penetration performance.
Homogeneity is also of cardinal importance in determining explosive composition performance. Homogeneity is dramatically influenced by local concentrations of residual air or gas, but is also influenced by local concentrations of binder (or other additives to the explosive powder), which often have specific gravities that are substantially different from that of the explosive powder.
Over the last two decades, attempts have been made to develop highly-filled, extrusion-molded or injection-molded energetic PBX for various applications. Addressing the problems associated with entrapped air is U.S. Pat. No. 4,555,277 to Scribner, entitled “Extrusion Cast Explosive”, which teaches a viscous mixture that is transferred within the system under a pressure exceeding atmospheric pressure. The use of viscous mixtures under pressure-deaeration enables a high solids loading of fine crystalline explosive.
It is further disclosed that specialized equipment needed to be developed for deaerating the mixture and for extrusion-casting the mixture to the desired shape. The equipment consists of a long, vertical, hollow cylinder with two hydraulic pistons operating from either end. Separating the two pistons is an orifice plate, below which are located vacuum and discharge ports. The mold or device for filling is connected to discharge port.
The mixed paste is placed in the upper chamber and the entire system is evacuated. The paste is forced through the orifice plate and thoroughly deaerated. The vacuum port is closed and, without breaking the vacuum, the material is forced into the mold while the mold is evacuated. Low pressure hydraulic cylinders that develop only 700-1400 Kpa are used.
The maximum filler content achieved by U.S. Pat. No. 4,555,277 to Scribner is 75.9 weight percent, which corresponds, after taking into account the specific gravity of each component, to only about 62 volume percent. It would be highly advantageous to have pastes having significantly higher filler contents.
U.S. Pat. No. 5,387,095 to Mahoney, et. al, entitled “Apparatus for Injection Molding High-Viscosity Materials”, discloses an apparatus for injection molding highly viscous materials in which incoming high-viscosity material flows through a supply conduit and is divided into strands by splitter plates as it flows into an evacuated chamber and is degassed. When the chamber is full, the supply of high-viscosity material is interrupted and the chamber is rotated to an injecting station while another chamber swings to the degassing station for filling. At the injecting station, a piston is lowered to engage the high-viscosity material in the first chamber, and a mold table is rotated to register a mold with a vacuum shroud and the mold is lifted into the shroud. The piston is further lowered in controlled increments to inject a quantity of high-viscosity material sufficient to fill a mold.
It is further disclosed by U.S. Pat. No. 5,387,095 that the flow passages for the high-viscosity material (at least about 8 kilopoise or 0.8 KPa·seconds) require a minimum orifice diameter of at least about 0.5 inch, in order to avoid flow stagnation and clogging of the high-viscosity material.
An explosive simulant disclosed by U.S. Pat. No. 5,387,095 to Mahoney, et. al, has a filler content of 90%, by weight. The stimulant consists of glass beads (90%) and polyurethane binder (10%), which is an idealization with respect to real fillers having particles with edges, coarse surfaces, etc. Even so, this explosive stimulant contains only 76 volume percent of the glass bead filler. Moreover, no information about the relative density (percent of TMD) of the paste is provided, nor measurements of paste homogeneity. However, given that the bead has a diameter of at least 0.5 inch, the deaeration is necessarily unsatisfactory for highly-filled pastes used in various high-performance applications.
It must be appreciated that improvements in filler content, even small improvements often result in appreciable increases in performance. It must be further appreciated that from a technological standpoint, such small improvements in filler content are extremely difficult to achieve. One reason behind this can be understood from FIG. 1, which graphically depicts, for a typical paste, the change in viscosity as a function of filler content (in volume percent). Up to a filler content of about 66%, the viscosity is almost constant, i.e., substantially insensitive to increased filler content. Above 66%, however, the viscosity rises rapidly. In terms of measured points: at a “baseline” filler content of 62.5 volume-%, the viscosity is 64.1 pascal·seconds (Pa·s). At a filler content of 67.5 volume-%, the viscosity is 297 Pa·s, which corresponds to an increase by a factor of 4.6 with respect to the baseline value. At a filler content of 70 volume-%, the viscosity is 2690 Pa·s, which corresponds to a 42-fold increase with respect to the baseline value. Moreover, the small increase in volume-% of filler from 67.5 volume-% to 70 volume-% results in a nine-fold increase in the viscosity. Such significant, sharp increases in viscosity have a profound effect on the flow properties of the paste, and introduce new hurdles with respect to the relative density and the homogeneity of the paste.
Thus, while it is often of great advantage to have more highly-filled, high-viscosity, homogeneous pastes, than those known heretofore, there is no device for, and method of, effectively eliminating occluded air in such paste and delivering the paste into a container, while maintaining homogeneity of the filler and binder materials within the paste.
There is therefore a recognized need for, and it would be highly advantageous to have, a device for, and method of, de-aerating and injecting high-viscosity pastes, and, particularly, for de-aerating and injecting high-viscosity pastes for use in the fabrication of munitions. It would be of further advantage for such a method and device to function in a simple, inexpensive and robust fashion so as to improve the power, range, precision and efficiency of the munitions.